Liquid resin infusion (LRI) is a process used to manufacture fiber-reinforced composite articles and components for use in a range of different industries including the aerospace, transport, electronics, building and leisure industries. The general concept in LRI technology involves infusing resin into a fiber reinforcement, fabric or a pre-shaped fibrous reinforcement (“preform”) by placing the material or preform into a mold (two-component mold or single-sided mold) and then injecting resin under high pressure (or ambient pressure) into the mold cavity or vacuum bag sealed single-sided mold. The resin infuses into the material or preform resulting in a fiber-reinforced composite article. LRI technology is especially useful in manufacturing complex-shaped structures which are otherwise difficult to manufacture using conventional technologies. Variation of liquid resin infusion processes include, but are not limited to, Resin Infusion with Flexible Tooling (RIFT), Constant Pressure Infusion (CPI), Bulk Resin Infusion (BRI), Controlled Atmospheric Pressure Resin Infusion (CAPRI), Resin Transfer Molding (RTM), Seemann Composites Resin Infusion Molding Process (SCRIMP), Vacuum-assisted Resin Infusion (VARI) and Vacuum-assisted Resin Transfer Molding (VARTM).
Since most resin infusion systems are inherently brittle, the viscosity levels necessary to achieve the injection process preclude the use of toughening agents. Said differently, the properties of toughness and low viscosity are mutually exclusive in conventional resin infusion systems. Addition of such tougheners to LRI systems generally results in an unacceptable increase in the viscosity of the resin and/or reduction in resistance of the cured material to solvents. These limitations render the addition of tougheners conventionally added in prepregs generally unsuitable in LRI applications.
One method to increase the toughness in composite articles manufactured by liquid resin infusion processes involves the use of non-woven veils of resin-soluble thermoplastic interposed between plies of dry structural reinforcement fiber. The veil may be comprised of a random mat of continuous or chopped polymer fibers. The fibers may be yarns or monofilaments of spun strands. When interleafed with one another, the layers of plies and veils form a preform. When the preform is positioned in a mold and injected with a curable resin, the resin-soluble thermoplastic veil at least partially dissolves throughout resulting in a toughened composite article.
Prior art resin-soluble thermoplastic veils are known to suffer from various shortcomings including bulkiness, low strength, uneven fabric areal weight (FAW) and premature dissolution. Variable uniformity of the FAW and certain characteristics of the fibers comprising the veil, e.g., fineness, directly affect the rate of dissolution of the fibers as well as the distribution evenness of the toughening agent in the composite. Bulkiness affects composite manufacture as well as composite cure ply thickness (CPT).